Protected pattern mask for reflection lithography in the extreme UV or soft X-ray range

ABSTRACT

A mask (MM) with patterns (MF) for use in a reflection lithography device with a photon beam with a wavelength of less than about 120 nm. Said mask (MM) comprises a planar substrate (ST) fixed to a reflecting structure (SMR) comprising a front face provided with selected patterns (MF) made from a material which is absorbent at the given wavelength and further comprises protection means (SP) which are transparent to the given wavelength and arranged such as to maintain a distance (H) between the perturbing particles (PP) and the patterns (MF) greater than or equal to one of the values of the depth of focus of the lithographic device and the height associated with the percentage of photon absorption by the perturbing particles (PP) which is acceptable.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of InternationalApplication Number PCT/FR2005/000168, filed on Jan. 26, 2005, whichclaims priority of French Patent Application Number 0400907 filed onJan. 30, 2004.

The invention relates to the field of patterned masks used in opticallithography.

Optical lithography is a well-known art allowing the reproduction on aresin layer, deposited on a substrate (or “wafer”) of patterns presenton one of the faces of a mask by means of a beam of photons and anoptical projection device, usually operating by reduction.

As the person skilled in the art knows, the resolution of the outlinesof patterns formed (or “insolated”) in resin is proportional to acritical dimension CD equal to kλ/NA, where λ is the wavelength of thephotons of the beam, k is a coefficient less than 1 representing theeffect of the devices used to reduce the theoretical limits (such as forexample the non-linearity of the resin), and NA is the digital apertureof the beam of photons at the patterns. In other words, the dimensionsof the patterns reproduced depend on the wavelength of the photons used.

Due to the properties and advantages offered by electronic components ofvery small dimensions, in order to realise these, ever smallerwavelengths are being used. Thus the transition has been made fromphotons having a wavelength in the visible spectrum (436 nm) to photonshaving a wavelength in the near ultraviolet spectrum (UV) (365 nm or 248nm), then to photons having a wavelength in the extreme ultravioletrange (193 nm or 157 nm), by using G and I rays of mercury respectively,then lasers with excimers KfF, ArF and F₂.

Since masks are difficult and particularly expensive to manufacturewithout defects, they must therefore be protected in order to avoid“interference” by ambient particles. Such protection is relativelysimple to implement when the mask is transparent to photons used forreproducing its patterns. In this case, the mask may in fact be used intransmission, such that it is possible to place a fine protectivemembrane of transparent, non-interfering material, e.g. polymer, infront of its front face (where the patterns to be reproduced arelocated) in order that the “interfering” (or unwanted) particles arekept apart from the patterns, thus preventing them from being reproducedon the resin (typically at a distance of 6 mm). This protective membrane(or film) can be cleaned, and in some cases may be replaced afterinspection.

When the limits of transparency of the mask are approached, e.g. for awavelength of 157 nm, the protection of its patterns is more difficult.First, the optical trajectory must be purged with nitrogen (N₂) in orderto remove gases, such as oxygen (O₂), molecules such as water (H₂O), andabsorbent polymers. Then, a solid protective film must be placed infront of the patterns, being composed of special quartz SiOF or CaF₂,which is transparent at 157 nm, having an antireflective treatment,having very parallel faces and a thickness selected such that it formspart of the optical calculation of the image formation of the patternson the resin.

When the wavelength of the photons is in the extreme ultraviolet (UV)range, or even soft X-ray range (typically between about 120 nm andabout 1 nm), the mask is no longer transparent, such that it must beused for reflection. Such a mask is then formed of a planar substratefixed to a structure which is reflective to the wavelength of thephotons being used (realised for example for a wavelength of 13.5 nm(located in the extreme ultraviolet range), in the form of amultilayered structure formed of alternating layers of silicon (Si) andmolybdenum (Mo) and having a front face equipped with selected patterns,formed from a material which is absorbent to the said wavelength (e.g.of Cr or TaN).

For this type of mask used in lithography by reflection, there is noknown means of protecting the patterns.

The object of the invention is therefore to overcome this disadvantage.

It proposes to this end a patterned mask, for a lithography deviceoperating by reflection of a beam of photons of a wavelength (λ) lessthan about 120 nm (and with a selected digital aperture (NA) at thepatterns), comprising a planar substrate connected to a reflectivestructure having a front face equipped with selected patterns formed ina material which is absorbent to the wavelength (λ).

This mask is characterised in that it comprises protective meanstransparent to the wavelength and contrived so as to keep interferingparticles at a distance (H) from the patterns which is greater than orequal to one of the values taken by the depth of focus (doF) of thelithography device and the height (h) which is associated with thetolerated percentage of absorption of photons by the interferingparticles.

For example, the protective means may be contrived such as to keep theinterfering particles at a distance (H) from the patterns which isgreater than or equal the greater of the values taken by the depth offocus (doF) of the lithography device and the height (h).

This distance (H) is for example between about 50 nm and about 5000 nm.However, it may be greater when a wavelength is used which moves awayfrom the range of soft X-rays.

The protective means of the mask may form a structure having additionalfeatures which can be taken separately or in combination, and inparticular it is preferable that:

-   -   The structure has a maximum variation of optical thickness        selected so as to bring about a local deflection of the beam        which is negligible compared to the precision of placing of the        patterns,    -   The structure does not bring about any phase variation between        the photons of the beam which are reflected by the mask,    -   The structure is hydrophobic,    -   The structure has a front face, opposite to the patterns, which        can be cleaned of at least some of the particles which it holds,    -   The structure is contrived so as to be capable of inspection,        with a selected contrast, by means of observation means        operating in the visible or in the ultraviolet (UV) range,    -   The structure is capable of thermophoresis,    -   The structure is conductive so as to allow the application of an        electrostatic effect, e.g. in order to repel the interfering        particles,    -   The structure is non-diffracting and non-diffusing in the        ultraviolet (UV) range.

However, this structure may take different forms, in particular:

-   -   It may be disposed on a front face of the reflective structure        and parallel thereto, and have at least one antireflective layer        formed from a selected material,    -   It may take the form of a foam of a selected material,    -   It may be formed from a selected material, be disposed on the        front face of the reflecting structure, and define channels        allowing its density to be reduced,    -   It may have a membrane connected by pillars to the front face of        the reflective structure and in a position substantially        parallel to this front face, the thickness of the membrane and        the height of the pillars being then selected such that their        sum is equal to the selected distance,    -   It may be composed of nanotubes, e.g. oriented in a selected        direction relative to the normal to the front face of the        reflective structure.

Among the materials which can be selected to form the structure,polymers transparent to the wavelength (λ, e.g. equal to 10.9 nm or 13.5nm), carbon (C), carbon nanotubes (or CNT), silicon (Si), beryllium(Be), ruthenium (Ru), silver (Ag), and zirconium (Zr) may be cited.

Further features and advantages of the invention will appear from astudy of the following detailed description, and from the attacheddrawings in which:

FIG. 1 shows schematically an embodiment of a device for lithography byreflection in the extreme ultraviolet (EUV) range,

FIG. 2 shows schematically, in a transverse section view, a firstembodiment of a patterned mask according to the invention,

FIG. 3 is a diagram showing the evolution of the parameter h accordingto the diameter d of the interfering particles, for two different valuesof absorption of photons (1% and 40%).

FIG. 4 shows schematically in a transverse section view a secondembodiment of a patterned mask according to the invention,

FIG. 5 shows schematically in a transverse section view a thirdembodiment of a patterned mask according to the invention,

FIG. 6 shows schematically in a transverse section view a fourthembodiment of a patterned mask according to the invention,

FIG. 7 shows schematically in a transverse section view a fifthembodiment of a patterned mask according to the invention.

The attached drawings may serve not only to complement the invention,but also to contribute to its definition if need be.

The invention relates to a patterned mask intended to be used in adevice for lithography by reflection operating with a source of photonswhose wavelength (λ) is less than about 120 nm, i.e. which belongs tothe extreme ultraviolet (EUV) range and to the range of soft X-rays, inparticular 10.9 nm and 13.6 nm.

We refer first to FIG. 1 to describe an embodiment, very schematically,of a device for lithography by reflection using a patterned mask MMaccording to the invention.

A lithography device chiefly comprises an optical image-forming deviceM3-M8 (also known as projection device) and a source S of photons,coupled to collecting mirrors M1 and M2, installed in an ultrahighvacuum enclosure E, in which are defined, very precisely, a positioningzone of a patterned mask MM and a positioning zone of a “wafer” W.

The patterned mask MM is intended to act reflectively. It will bedescribed in more detail below.

The wafer W is generally formed of a planar substrate equipped on one ofits faces with a layer of resin R sensitive to the photons delivered bythe source S of the lithography device.

The source S is for example responsible for delivering photons whosewavelength λ is equal to 13.5 nm. Such a wavelength is for exampleobtained with a discharge or plasma laser source in xenon (Xe) or tin(Sn). However, obviously, it could deliver photons having otherwavelengths between about 120 nm and about 1 nm, and in particular awavelength equal to 10.9 nm (which corresponds to another range ofemission of xenon (Xe).

In the example shown, the mirrors M1 and M2 and the filter FR areresponsible for the collimation of the photons delivered by the sourceS, so that they reach the front face of the mask MM (which has thepatterns MF (see FIG. 2)) in the form of a beam having a selecteddigital aperture NA_(i). For example, NA_(i) is equal to 0.064, whichcorresponds to an angle of aperture on the patterns of ±3.6°.

However, the optical image-forming device M3-M8 (also known as opticalprojection device) is responsible for forming the image of the patternsMF of the mask MM on the resin R of the wafer W, with a selected factorof reduction, e.g. equal to about 4. It is formed here of six mirrors M3to M8, by way of example.

The angles of incidence of the beam of photons on the different mirrorsand the respective positions of the different mirrors are selected so asto allow a factor of reduction to be obtained as well as an illuminationof the layer of resin R under a selected digital aperture NA_(f), e.g.equal to about 0.25.

The angle of incidence α of the beam of photons relative to the normal Nto the front face of the patterned mask MM (see FIG. 2) is generallyequal to a few degrees, e.g. about 6°.

Obviously, this is only a very schematic illustrative example. Numerousother combinations of optical means can be conceived to effectcollimation and image formation.

We refer now more particularly to FIG. 2 in order to describe a firstembodiment of the patterned mask MM according to the invention. In thisFigure, as in FIGS. 4 to 7, the relative dimensions of the differentelements are not representative of their real relative dimensions.

A patterned mask MM, used reflectively, first of all has a planarsubstrate ST, one face of which is connected to a structure SMRreflective to the wavelength λ of the photons from the source S andcomprising a front face equipped with selective patterns MF formed froma material absorbent to the wavelength λ.

For example, the reflective structure SMR is a multilayered structureformed of a stack of 40 pairs of layers of silicon (Si), e.g. 4 nmthick, and of molybdenum (Mo), e.g. 2.7 nm thick.

Buffer layers and protective layers can be added for technologicalreasons.

The absorbent patterns are for example formed from chromium (Cr) or TaN.However, any other material absorbent to the wavelength λ of the photons(here equal to 13.5 nm) is conceivable. The thickness of the patterns MFis preferably reduced so as to avoid the edge effects of the masks whosedimensions are typically of the order of 152 mm×152 mm (for a zone of104 mm×104 mm reserved for the patterns MF). However, taking intoaccount a reduction factor of about 4, the lines printed in the layer ofresin R have for example a width of 25 nm, 32 nm or 45 nm, whichcorresponds to patterns MF whose widths are respectively 100 nm, 128 nmand 180 nm.

As the person skilled in the art knows, and as referred to in theintroduction, numerous interfering particles PP of very small dimensions(e.g. 30 to 60 nm) are capable of infiltrating between the absorbentparts forming the patterns MF, thus impairing the integrity of the maskMM. The interfering particles PP do not tangibly obstruct either theimage formation nor absorption when they are located on the absorbentparts.

In order to prevent such infiltration from arising, the inventionproposes to place in front of the patterns MF protective means SPtransparent to the wavelength λ of the photons and having the task ofkeeping the interfering particles PP at a distance H from the patterns,which is greater than or equal to one of the values taken by the depthof focus doF of the lithography device and the height h associated withthe tolerated percentage of absorption of the photons by the interferingparticles PP.

The depth of focus doF is equal to λ/NA_(i) ² For example, in the caseof a digital aperture NA_(i) equal to 0.064 and a wavelength λ equal to13.5 nm, a depth of focus doF equal to about 3296 nm is obtained.Obviously, this is only an example, and doF may vary typically betweenabout 50 nm and about 5000 nm according to the selected values forNA_(i) and λ, or even more when a wavelength is being used which ismoving away from the range of soft X-rays.

Furthermore, it should be borne in mind that the percentage ofabsorption of photons by the interfering particles PP is defined by thefactor (or percentage) of shade which is given by the formula:

${{Percentage}\mspace{14mu}{of}\mspace{14mu}{shade}} = ( \frac{d}{h^{*}2{NA}_{i}} )^{2}$where d is the diameter of the interfering particles PP, and h is theparameter representing the height separating the interfering particlesfrom the patterns MF.

If an absorption of photons equal to about 1% is tolerated, thesimplified relation h=80*d is obtained. On the other hand, if anabsorption of photons equal to about 4% is tolerated, the simplifiedrelation h=40*d is obtained. The diagram of FIG. 3 shows the evolutionof the parameter h (in nanometers (nm)) as a function of diameter d (innanometers (nm)) of the interfering particles for photon absorptionvalues equal to 1% (upper curve) and 4% (lower curve).

The smaller the diameter d of the interfering particle PP, the less theparameter h occurs in the choice of distance H separating the front partof the protective means SP from the front face of the reflectivestructure SMR (where the patterns MF are formed). The value of H musttherefore be selected to be greater than or equal to one of the valuestaken by doF and h.

For example, in the first order, the following conditions can be fixed:if doF is greater than h, the distance H must be greater than or equalto doF, whereas if doF is less than h, the distance H must be greaterthan or equal to h. In other words, in the first order, the value of His selected greater than or equal to the greatest of the values taken bydoF and h. Simulation calculations of image formation taking intoaccount the additional parameters, such as for example diffractionand/or the differences in index, make it possible to refine theabove-mentioned conditions.

The protection means SP can further have one or more additional featureswhich may reinforce the performance and/or advantages.

It is for example advantageous that the protective means SP absorb veryfew, if any, photons. It should be borne in mind particularly that inreflection lithography, the optical thickness T of the protective meansSP is traversed twice. To this end, it is preferable to use a materialwith low, i.e. very low, ageing, resistant to the beam of photons andhaving low, i.e. very low oxidation.

It will be noted that an interfering particle PP can be traversed twice,but not by the same beam.

It is for example also advantageous that the protective means SP have amaximum variation of optical thickness T which brings about localdeflection of the beam of photons which is negligible in relation to theprecision of placing of the patterns MF.

It is for example advantageous that the protective means SP do not bringabout (or hardly bring about) any phase variation between the photons ofthe beam which are reflected by the mask MM.

It is for example advantageous that the protective means SP arehydrophobic. It is to be borne in mind that water molecules (H₂O) areabsorbent to 13.5 nm.

It is also advantageous for example that the front face of theprotective means SP, which is opposite to the patterns MF, can becleaned of at least some of the interfering particles PP which it holds.In this case, it is preferable that the protective means SP can beinspected, with a selected contrast, by observation means operating inthe visible or ultraviolet range, e.g. at 248 nm.

Instead of cleaning the front face of the protective means SP, it isconceivable to remove them from the mask MM in order to replace them.This removal can be effected for example by combustion or oxidation(particularly if they are formed from a base of carbon nanotubes, aswill be seen below) by means of carbon dioxide (CO₂, which is thenevaporated and pumped in order to avoid residual deposits. Once thisremoval has been carried out, the mask MM can be inspected, then newprotective means SP can be placed on the front face.

It is for example also advantageous that the protective means SP arecapable of thermophoresis.

It is for example also advantageous that the protective means SP areelectrically conductive in order to be used to apply an electrostaticeffect, e.g. in order to repel the interfering particles when they areionised.

It is for example also advantageous that the protective means SP arenon-diffracting and non-diffusing in ultraviolet (UV), including inextreme ultraviolet, for the quality of the image, in particular whenthe wavelength is equal to 13.5 nm, and for the quality and contrastupon inspection.

Numerous structures may constitute the protective means SP indicatedabove. These structures must always be fixed to the substrate ST or tothe reflective structure SMR and may comprise a part for keeping at adistance the interfering particles PP, also known as a film (ormembrane), which is either remote from the patterns MF or in contacttherewith.

As is shown schematically in FIG. 1, the protective means SP may consistin a planar antireflective structure, preferably of the multilayeredtype, placed on the front face of the reflective structure SMR andparallel thereto. To this end, Mo—Si layers may be used for example.

In a first modification, shown in FIG. 4, the protective means SP mayform a structure composed of nanotubes oriented in a selected directionrelative to the normal N to the front face of the reflective structureSMR. For example, carbon nanotubes (CNT) may be used, preferably havingwalls of a monoatomic thickness, spaced apart by a distance less thanthe diameter of the smallest interfering particles PP, and being capableif necessary of having a disorientation relative to the normal N.

In a second modification, shown in FIG. 5, the protective means SP mayform a structure having layers CL in which are defined channels CX ofdimensions adapted relative to those of the interfering particles PP,with a view to blocking these at their front ends. Since these channelsCX are filled with a “vacuum”, they make it possible to reduce thedensity of the material. Such a structure may be defined by means of alithography technology applied to a material such as silicon (Si) or apolymer, such as for example PMMA. It may also be obtained by adding tothe front face of the reflective structure SMR a photonic crystal orporous silicon.

In a third modification, shown in FIG. 6, the protective means SP mayform a foam structure forming a membrane (or film), preferably connectedto the front face of the reflective structure SMR. As in the secondmodification, the foam contains numerous empty spaces which make itpossible to reduce the density of the material. This foam may be forexample formed from a polymer, such as PMMA, or of nanotubes, e.g. ofcarbon (or CNT), or of beryllium (Be), ruthenium (Ru), silver (Ag) oreven zirconium (Zr).

In a fourth modification shown in FIG. 7, the protective means SP mayform a structure formed by a membrane (or film) ME connected by pillarsPS to the front face of the reflective structure SMR. Some ends ofpillar PS may bear on the patterns MF, as is shown. The membrane ME isplaced substantially parallel to the front face of the reflectivestructure SMR. However, the thickness of the membrane ME and the heightof the pillars PS are selected such that their sum is equal to theselected distance H. The pillars may be obtained for example by growingseeds. Such a structure may be realised for example from a polymer, suchas PMMA, or from silicon (Si), or even from an antireflective materialsuch as Mo—Si. The material forming this structure may also be presentin the form of a foam, as in the third modification indicated above withreference to FIG. 6.

The invention is not limited to the embodiments of a patterned maskdescribed above solely by way of example, but also embraces all themodifications which the person skilled in the art may conceive withinthe scope of the claims below.

1. Mask (MM) having patterns (MF), for a lithography device operating byreflection of a beam of photons of a wavelength less than about 120 nm,comprising a planar substrate (ST) connected to a reflective structure(SMR) comprising a front face equipped with selected patterns (MF),formed from a material absorbent to the said wavelength, characterisedin that it comprises protective means (SP) contacting the reflectivestructure (SMR) and transparent to the said wavelength and contrived tokeep interfering particles (PP) at a distance (H) from the patterns (MF)which is greater than or equal to one of two values taken from a depthof focus (doF) of the device and a height of pattern/interferingparticle (h) associated with a tolerated percentage of absorption ofphotons by the interfering particles (PP) which is a function of theirdiameter (d), and characterised in that the protective means (SP) form astructure having a membrane (ME) connected by pillars (PS) to the frontface of the reflective structure, and in a position substantiallyparallel to the front face, the thickness of the membrane (ME) and theheight of the pillars (PS) being such that their sum is equal to theselected distance (H), the pillars (PS) contacting at least one of thepatterns (MF).
 2. Mask according to claim 1, characterised in that theprotective means (SP) are contrived to keep the interfering particles(PP) at a distance (H) from the patterns (MF) which is greater than orequal to the two values taken by the depth of focus (doF) of the deviceand the height of pattern/interfering particle (h).
 3. Mask according toeither of claim 1 or 2, characterised in that the protective means (SP)form a structure having a maximum variation of optical thicknessselected so as to bring about locally a deflection of the beam which isnegligible compared to the precision of placing of the patterns (MF). 4.Mask according to claim 1, characterised in that the protective means(SP) form a structure which brings about substantially no phasevariation between photons of the beam reflected by the mask.
 5. Maskaccording to claim 1, characterised in that the protective means (SP)form a hydrophobic structure.
 6. Mask according to claim 1,characterised in that the protective means (SP) form a structure ofwhich at least a front face, opposite to the patterns (MF), is capableof being cleaned of some at least of the interfering particles (PP)which it holds.
 7. Mask according to claim 1, characterised in that theprotective means (SP) form a structure capable of being inspected, witha selected contrast, by means of observation means operating in thevisible or ultraviolet range.
 8. Mask according to claim 1,characterised in that the protective means (SP) form a conductivestructure capable of thermophoresis.
 9. Mask according to claim 1,characterised in that the protective means (SP) form a conductivestructure capable of applying an electrostatic effect.
 10. Maskaccording to claim 9, characterised in that the electrostatic effect isintended to repel the interfering particles (PP).
 11. Mask according toclaim 1, characterised in that the protective means (SP) form astructure which is non-diffracting and non-diffusing in the ultravioletrange.
 12. Mask according to claim 1, characterised in that the distance(H) is between about 50 nm and about 5000 nm.
 13. Mask according toclaim 1, characterised in that the protective means (SP) form astructure placed on the front face of the reflective structure andparallel thereto, and comprising at least one antireflective layer. 14.Mask according to claim 1, characterised in that the protective means(SP) form a structure composed of a foam.
 15. Mask (MM) having patterns(MF), for a lithography device operating by reflection of a beam ofphotons of a wavelength less than about 120 nm, comprising a planarsubstrate (ST) connected to a reflective structure (SMR) comprising afront face equipped with selected patterns (MF), formed from a materialabsorbent to the said wavelength, characterised in that it comprisesprotective means (SP) contacting the reflective structure (SMR) andtransparent to the said wavelength and contrived to keep interferingparticles (PP) at a distance (H) from the patterns (MF) which is greaterthan or equal to one of two values taken from a depth of focus (doF) ofthe device and a height of pattern/interfering particle (h) associatedwith a tolerated percentage of absorption of photons by the interferingparticles (PP) which is a function of their diameter (d), andcharacterised in that the protective means (SP) form a structure formedfrom a material, placed on the front face of the reflective structure(SMR) and defining channels (CX) making it possible to reduce thedensity of the material, at least one of the patterns (MF) beingaccessible to the channels.
 16. Mask (MM) having patterns (MF), for alithography device operating by reflection of a beam of photons of awavelength less than about 120 nm, comprising a planar substrate (ST)connected to a reflective structure (SMR) comprising a front faceequipped with selected patterns (MF), formed from a material absorbentto the said wavelength, characterised in that it comprises protectivemeans (SP) contacting the reflective structure (SMR) and transparent tothe said wavelength and contrived to keep interfering particles (PP) ata distance (H) from the patterns (MF) which is greater than or equal toone of two values taken from a depth of focus (doF) of the device and aheight of pattern/interfering particle (h) associated with a toleratedpercentage of absorption of photons by the interfering particles (PP)which is a function of their diameter (d), and characterised in that theprotective means (PS) form a structure composed of nanotubes oriented toextend along a selected direction substantially normal (N) to the frontface of the reflective structure (SMR), the nanotubes contacting atleast one of the patterns (MF).
 17. Mask according to claim 13,characterised in that the at least one antireflective layer is composedof a material selected from at least polymers transparent to the saidwavelength, carbon, carbon nanotubes, silicon, beryllium, ruthenium,silver or zirconium.